Look at /mnt/custom/customcd/isoroot/isolinux/isolinux.cfg and you will find the 64 bit kernels all replaced by setkmap=us and the 32 bit branch with no keymap set. Needless to say this no longer boots in a 64 bit kernel (as setkmap=us isn't a kernel)!

Fix:

I'm not entirely sure in this case, but I think something that may work is after removing any existing setkmap entries replace the "--" with "setkmap=us -- setkmap=us". This does two things, sets the keymap for both 64 and 32 bit kernels and as importantly does't attempt to set a keymap for the non rescuecd entries now in the isolinux.cfg file such as memtest (as there isn't a "--" to replace). You will need to remember to not use "--" in non keymap using entries though and may see a better way to fix this.